1. Field of the Invention
The present invention relates to a method for manufacturing toner used in image formation by electrophotographic method.
2. Description of the Related Art
Generally, a toner used for image formation by electrophotographic method is obtained in the manner described below. Firstly, coloring agent, bonding resin, and other additives are mixed and kneaded. Then, the kneaded material is coarsely pulverized, subsequently finely pulverized, and finally classified.
Various means for finely pulverizing the material which has been coarsely pulverized are well known. Examples such means are crushing machines which pulverize via a mesh and cutter that rotates at high speed, hammer mills and turbo mills which shear via the spacing of a liner and a hammer that rotates at high speed, pin mills which shear via the high speed rotation of pins that protrude onto a disk, kryptron pulverizers which shear and pulverize by friction between a large rotor and a liner, jet mills which pulverize by impingement on an impact plate via jet air injected in the pulverized material and the like.
Conventionally, when manufacturing toner having, for example, a mean particle diameter of about 12.about.15 .mu.m, a jet airflow type pulverizer is used to finely pulverize coarsely pulverized material to desired small-diameter particles without any step.
On the other hand, today ever finer particle diameter toner is needed to produce a copy image of high image quality. Certain disadvantages arise, as described hereinafter, when the previously mentioned methods and apparatus are used to manufacture fine diameter particles having a mean particle diameter of 10 .mu.m or less and said toner is used, for example, in a two-component developer comprising a toner and a carrier.
When toner manufactured by the previously mentioned methods and apparatus is used as, a two-component developer, fine powdered toner (binder resin, charge controlling agent and the like) resulting from excessive pulverization becomes mixed in said developer. In developing devices using a two-component developer, the toner is charged by mixing and stirring the carrier and toner. During this mixing and stirring, the aforesaid fine powder toner adheres to the surface of the carrier. The adhesion of this fine powder toner is disadvantageous inasmuch as it causes deterioration of carrier chargeability, and interferes with durability of the developer.
In general, toner having a small particle diameter has a high probability of contact with other toner particles (surface area), thereby causing a further disadvantage of reduced heat resistance due to diminished flow characteristics.
When ultra pulverization produces a mean particle diameter of 10 .mu.m or less, the spread of particle distribution increases, thereby producing a plurality of fine polar particles. These fine polar particles may electrostatically adhere to the surface of the toner particles having a particle diameter of 10 .mu.m, and readily remain on the product even after air classification. These residual fine polar particles readily cause of a further disadvantage of fog phenomenon during image formation.
Furthermore, when fine particle toner having a mean particle diameter of 10 .mu.m or less is manufactured via the aforesaid methods and apparatus, much energy is required to achieve such a degree of pulverization, thereby diminishing production capacity, and ultimately increasing the cost of the product.